1. Field of the Invention
This invention relates generally to medical systems for the treatment of atrophied or spastic muscles. More particularly, the present invention relates to a system and method for electrically stimulating muscles during sleep.
2. General Background and State of the Art
There are many medical conditions for which it is desirable but difficult or impossible to exercise particular muscles to build their strength or reduce spasticity. Medical conditions such as stroke, spinal cord injury and cerebral palsy result in paralyzed, weakened and/or spastic muscles as a result of deficits of descending control from the brain to the spinal cord. Weakness, atrophy and imbalances of muscle function occur also as secondary problems of joint pathology such as arthritis in which pain inhibits normal muscle use. Electrical stimulation can be used to strengthen such muscles, preventing disuse atrophy and correcting spasticity and contractures that result from imbalances in antagonistic pairs of muscles. Electrical stimulation excites the peripheral axons of motoneurons, which in turn activate the muscle fibers that they innervate via synaptic connections. The resulting activation of muscle fibers exerts effects similar to normal voluntary exercise, improving the strength and fatigue resistance of the stimulated muscles and stretching spastic antagonist muscles, preventing contractures. These useful effects of exercise are well recognized by sport coaches, athletic trainers, physical therapists and practitioners of sports medicine and exercise physiology, although the trophic physiological mechanisms underlying such effects are not well understood.
Electrical stimulation tends to excite any large diameter sensory axons that happen to be in the vicinity, as well as the motor axons that are the usual target of therapeutic electrical stimulation. When the stimulating current is applied via electrodes within the muscle or on a muscle nerve, the nearby large diameter sensory axons arise mostly from proprioceptors. The reflex effect of such proprioceptive activation tends to excite further the motoneurons controlling the stimulated muscle and its synergists. Other reflex effects of such proprioceptive activation tend often to inhibit the activity of motoneurons controlling antagonist muscles. Both of these are useful effects, both acutely to enhance the effects of the motor stimulation and chronically to reduce hyper-reflexive spasticity. In contrast, when the stimulating current is applied through the skin or onto mixed peripheral nerve trunks containing both muscular and cutaneous components, the large diameter axons that are activated will include the mechanoreceptors of the skin. Such activation produces a variety of reflex actions, often inhibiting voluntary muscle function. It also gives rise to various conscious sensations that may be distracting or unpleasant depending on the site, strength and duration of the stimulation.
Some devices comprising microstimulator implants themselves and for certain applications including therapeutic electrical stimulation (TES) have been developed. This application contains reference to U.S. Pat. Nos. 6,051,017, 6,061,596, 5,312,439, 5,405,367, 5,193,539, 5,193,540, and 5,324,316 which describe microstimulator implants, each of which is hereby incorporated by reference in their entirety.
U.S. Pat. No. 6,240,316 describes the use of microstimulators for the treatment of sleep apnea in which the transmission coil is located under the sleeping patient, but this is a form of neuromodulatory stimulation rather than therapeutic stimulation. In neuromodulatory stimulation (NMS), the stimulation pulses themselves are used to create a desired physiological response; a common example is a cardiac pacemaker. In TES, the stimulation pulses are used to exercise the muscles and neural pathways so as to induce long-term changes in their structure and function according to trophic principles whereby various body tissues adjust their properties in response to the demands of regular usage patterns. This invention is particularly applicable to TES applications, which generally require fairly intense stimulation over substantial periods of time, often delivered to multiple sites.
The sleep apnea patent describes the use of a single transmission coil, which is easily located in a collar or pillow that moves with the patient during sleep. The switched transmitter coil technology described in this application is particularly important when stimulating limb or trunk muscles, particularly in children who tend to move a lot during their sleep, but it may also be useful for the sleep apnea application. The sleep apnea application was presented in two forms: one with back-telemetry of signals from a sensor of respiratory movement and one with stimulation only and no back-telemetry, which substantially simplifies the system. This invention requires that back-telemetry be available to track the position and responses of the implanted stimulators but it does not require a separate sensing function.
Other relevant technology can be found in the area of transcutaneous electrical nerve stimulators (TENS) and percutaneous wire electrodes injected into muscles and used for functional electrical stimulation (FES). An implanted system for FES of the arm called FreeHand(trademark) has been marketed by NeuroControl Corp. of Cleveland, Ohio. It is controlled by an inductive RF link similar to a cochlear implant, with an external transmission coil designed for close coupling; it is not intended or suitable for use while sleeping. A stimulator for strengthening shoulder muscles that employs fine wires inserted into the target muscles and passing physically through the skin for connection to a more conventional electrical stimulator is presently being tested by the same company. Such a system would be possible, albeit awkward, to use during sleep, but its vulnerable percutaneous leads make it unsuitable for long-term use. Transcutaneous stimulation (electrodes attached adhesively to the surface of the skin) has been used to condition muscles in patients at rest, but the high stimulus strength required to activate deep muscles invariably produces strong cutaneous sensations which are usually unpleasant and likely to interfere with sleep.
In order to obtain useful therapeutic results in muscles, it can be necessary to apply electrical stimulation for several hours per day over many months. Thus it is important to minimize the disruptive effects of the therapy, particularly for younger and older patients who may be reluctant to comply with such prescribed treatment. Disruptive effects include the time and attention required to set up stimulation equipment, time taken from other activities of daily living to receive the treatment, and any unpleasant sensations associated with the electrical stimulation. It is also important to minimize the overall cost of chronic treatment and to minimize possible medical complications such as damage to skin and muscles, as well as wound infections and morbidity that may arise from surgical procedures.
Therapeutic electrical stimulation (TES) of muscles is most commonly applied by the transcutaneous approach, in which electrodes are affixed to the skin overlying the so-called motor point of the muscles to be electrically exercised. This is the region where the muscle nerve enters the muscle, in theory permitting relatively selective and complete activation of the muscle. In reality, quite high voltages and currents are required to activate motor axons that pass deep to the skin surface, resulting in substantial activation of more superficial cutaneous nerves. Even when accurately positioned by an experienced therapist, such electrodes often fail to activate completely the target muscles, particularly if they are located deep to the skin surface. At each treatment session, stimulation levels must be adjusted carefully to maximize the desired therapeutic effects and minimize unpleasant sensations. The therapist must watch for skin irritation that may be produced by the stimulation, particularly if the electrodes are affixed poorly or become dislodged during use. For these reasons, TES is used only infrequently despite its potentially beneficial effects, and is generally deemed to be impractical for young patients with cerebral palsy and older patients with strokes for whom it would be particularly useful.
A less common approach to TES is percutaneous, intramuscular wire electrodes. Fine wires are injected into the target muscles through a hypodermic needle and remain chronically in place with their leads extending through the skin. When treatment is applied, the external ends of the leads are attached to a conventional electrical stimulator. This intramuscular approach provides selective stimulation of the motor neurons innervating the target muscle without exciting other muscle or skin nerves. The required stimulation currents tend to remain constant as long as the electrode tips are not dislodged from the muscle. The percutaneous wires pose significant dangers of infection and dislodgment, however, and are not suitable for long term use in most patients, particularly those already disabled by their underlying neurological problems.
Other approaches to electrical stimulation of muscle are known but are not generally suitable for chronic TES. Magnetic stimulation can induce electrical currents sufficient to activate motor axons, but this is even less selective and more difficult to apply than transcutaneous stimulation. Fully implanted stimulators similar to cardiac pacemakers have been used for functional electrical stimulation to reanimate paralyzed limbs, but the devices are expensive and they require extensive surgery to implant and route their leads to various target muscles.
Accordingly, an object of the present invention is to provide a method and apparatus for applying therapeutic electrical stimulation (TES) to specific muscles while a patient sleeps. It is a further object to permit patients themselves and/or their domestic caregivers to administer TES in an easy to use manner without requiring trained assistance for each treatment session.
The present invention teaches the application of electrical stimulation to activate such muscles via means suitable for use during sleep. The apparatus includes microminiature electrical stimulators that are injected into the muscles to be exercised and a system of transmission coils located in or on the mattress of a bed. The transmission coils transmit power and command signals to the implanted electrical stimulators while the patient sleeps or rests. The implanted electrical stimulators can be programmed so as to produce the desired pattern of muscle exercise without producing cutaneous sensations that would disturb the patient.
The invention includes two primary features. First is an intramuscular microstimulator (called a BION(trademark)), one or more of which are injected into the muscles to be exercised, where they generate stimulation pulses upon commands transmitted to them from an external controller. Additionally there is an external controller (called Personal Trainer(trademark)), which transmits power and command signals to the BIONs by means of one or more inductive coils placed in or on a mattress while the patient sleeps.
In one embodiment of the invention, the BIONs verify that they have received a given command signal by emitting a confirmatory signal that is detected via the inductive coil that transmitted the command. In the absence of such a confirmation, the Personal Trainer can switch the command signal to another transmission coil in order to accommodate motion of the patient during sleep.
The invention further includes a system for exercising muscles while a patient sleeps, comprising a microstimulator implanted into the body, an inductive coil placed in or on a surface upon which the patient rests, and a controller capable of transmitting power and commands to the microstimulator via one or more inductive coils external to the body.
The invention also includes a system for exercising muscles while a patient sleeps comprising a microstimulator capable of emitting a signal in response to a command, where the signal is detectable by a controller.
The invention additionally includes a system for exercising muscles while a patient sleeps comprising a controller capable of switching among a multiplicity of inductive coils so as to find at least one or more inductive coils which can be used successfully to transmit commands to a given microstimulator.